Biomedical sensors, such as enzyme electrodes, can be used to determine the concentration of certain biochemicals rapidly and with considerable accuracy. Enzyme electrodes can detect glucose, urea, uric acid, various alcohols, and a number of amino acids under certain well-defined conditions. For example, glucose sensors suitable for in vivo use can be prepared by depositing a glucose sensitive enzyme, such as glucose oxidase, onto an electrode via an electromotive plating process.
Sensor configurations currently in use require a minimum of one polymeric membrane at the interface with the in vivo environment. This membrane (external surface) serves two functions. First, the membrane limits diffusion, e.g. of glucose, while maintaining high oxygen permeability. Second, the membrane provides a biocompatible interface with the surrounding tissue.
The introduction of a material into the body, however, initiates protein fouling or deposition at the surface of the material or device. Following the deposition of protein at the surface, a new surface is essentially created. This new surface influences the temporal sequence of events associated with the healing process. In the context of a biosensor, shortly after the injury initiated by implantation of the sensor, monocytes arrive at the material surface and differentiate into macrophages soon thereafter.
Macrophages are potent generators of damaging chemicals that aid in the process of phagocytosis. These chemical entities and by-products can include hydroxyl radical, superoxide, and strong acids, which may diffuse through the membrane to the underlying enzyme layer.